In a nutshell, the new facility offers an interface to
many of the system calls and library
routines documented in chapters 2 and 3 of the UNIX manual.
Some, but not all, of the functionality is available on Microsoft Windows.
This is somewhat of a philosophical departure for us: we
strive to make each and every feature be available on all the platforms on
which Allegro Common Lisp is supported (16 platforms).
We felt for Lisp to be competitive in certain areas
we needed to break this rule.

In addition to the interface to system services,
we have also created three new process manipulation operators.
They can be used when input to and/or output from a subprocess is needed.
Below are some examples using the new operators
command-output,
with-command-output,
and
with-command-io.
One important thing to note about these new, higher-level process operators:
they handle the reaping of the subprocess exit status
(use of sys:reap-os-subprocess is not needed)
as well as closing the streams associated with input to and output
from the subprocess. When using
excl:run-shell-command these tasks are left to the user.

The new functionality is present in the osi module and
excl.osi package.